Make anchors mouse-focusable

Anchors are now also mouse-focusable, to be consistent with Gecko,
Trident and Presto. When an anchor is focused by mouse, the default
focus ring is not shown (unless tabindex is set), as before.

The expectation of fast/events/click-focus-anchor has changed as follows:
- a3 is also focusable by mouse, because it is a link.
- a7 is focused instead of p7 because a7 is focusable, and therefore get
  the focus instead of the parent <p> element.

BUG=388666

Review URL: https://codereview.chromium.org/455223002

git-svn-id: svn://svn.chromium.org/blink/trunk@181127 bbb929c8-8fbe-4397-9dbb-9b2b20218538